The present application relates generally to variable speed drives. The application relates more specifically to systems and methods in variable speed drives having active converters with integral ground fault protection.
A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications may include a rectifier or converter, a DC link, and an inverter. Ground fault protection within a VSD can be implemented in various ways. One such method utilizes an external ground fault sensor (a single “zero sequence” current transformer and detection circuitry) that opens a set of relay contacts. The relay contacts may be connected to a shunt-trip device within a circuit breaker that opens the circuit breaker when energized. The circuit breaker is placed at the input power terminals of the VSD. Another ground fault protection method utilizes a molded case circuit breaker with a trip unit that incorporates a ground fault detection circuit. These units typically have a higher level of ground fault current trip as they typically utilize three current sensors, one per phase on a three-phase system, and sum the outputs to effectively sense the zero sequence/ground fault current. The integrated circuit breaker units are much more compact than earlier ground fault circuit breakers. However the level at which the ground fault current trip can be sensed is greater, and the accuracy of the sensed ground fault current is reduced, as a result of the three phase sensors. Another ground fault protection method utilizes motor current sensing means to shut down the inverter section of the VSD. This method fails to provide ground fault protection in the event of a ground fault occurring internally in the VSD. This method also fails to provide  ground fault protection when there is a low impedance earth-referenced power feed and a ground fault occurs at the output of the VSD.